In boilers, heat exchangers, tube reactors, evaporators and condensers, so-called tube bundles are formed by groups of tubes and can terminate in tube bottoms or sheets which are plates. The plates may be secured in a housing which surrounds the tube bundle and can be traversed by the tubes providing both mechanical support and fluid-tight connection for the tubes.
It is known to form a fluid-tight press fit of a tube in a tube sheet or like plate, by inserting an end of the tube into a bore of the tube sheet with play or clearance and then expanding the tube end so that it bears uniformly against the wall of the bore for sealing and mechanical retention.
Mechanical expansion of the tube can be used for this purpose although, more recently, hydraulic expansion has become of greater interest.
In the hydraulic expansion technique a pressurizing mandrel is inserted into the tube end and the gap between the mandrel and the inner wall of the tube is sealed at two spaced-apart locations defining between them an expansion zone and a liquid is fed under high pressure into this zone to press the tube wall outwardly in the manner described.
Hydraulic expansion of the tubes is known from German patent document DE-OS No. 19 39 105 and in the article by M. Podhorsky and H. Krips entitled (in translation) "Hydraulic Expansion of Tubes" in VG Kraftswerktechnik, Number 1, 1979, pages 81-87.
The pressure-tight connection of tubes in tube sheets or the like, especially for heat exchanger tube bundles, represents a significant saving over earlier methods of securing such tubes and this has been recognized for several decades during which the mechanical expansion of tubes has been utilized.
However, even mechanical expansion is expensive and has the disadvantage that results are not always reproducible and it is not always possible to ensure that a particular tube attachment will correspond to a given standard. These problems are discussed in the Podhorsky et al article.
As noted by Podhorsky et al, hydraulic expansion has a number of advantages over the mechanical approach.
However, a practical problem arises with respect to the sealing of the opposite ends of the expansion zone in prior tube expansion techniques.
For example, as is described in German patent document DE-OS No. 19 39 105, the seals are generally applied after insertion of the mandrel by mechanical means, generally the use of compression cones which bear in the radial direction. This type of seal has the drawback that the setting of the seal by displacing at least one of the cones in the axial direction is time-consuming.
It is also a problem that at some applied pressure levels, the sealing effect may not be satisfactory.
In another hydraulic system described in German patent document DE-OS No. 24 00 148, the seal has a slightly larger diameter, before insertion of the mandrel in the tube end, than the inner diameter of the latter.
The insertion of the mandrel and the seal requires considerable axial force and effort and the sealing effect is not always satisfactory.
For example, since tolerance variations in the inner diameter and outer diameter of the tube ends and seals must be reckoned with, there are times when the maximum tolerance of the tube ends and the minimum tolerance of the seal may result in leakage and pressure loss.
Furthermore, because considerable axial force must be applied, damage to the seal may occur in the setting. Finally, since the seals can engage the inner wall of the tube tightly during insertion, trapped air, which cannot be vented, can be strongly compressed to impede insertion and pose problems with respect to the subsequent expansion.